1. Field of Invention
The present invention pertains generally to soil engineering and more particularly to retaining walls.
2. Discussion of the Background of the Invention
Various retaining wall systems have been developed for retaining soil on an embankment. Following patents are examples of retaining wall systems which have been developed over a number of years:
______________________________________ U.S. Pat. No. Inventor Date ______________________________________ British Patent Walter E. Adams Apr. 23, 1908 No. 1402 1,778,574 J. H. Thornley Oct. 14, 1930 1,909,299 H. B. Mette May 16, 1933 4,050,254 Meheen et al. Sept. 27, 1977 4,260,296 Hilfiker April 7, 1981 4,384,810 Newmann May 24, 1983 ______________________________________
In conventional retaining wall design, one of the major design criteria is the pressure exerted on the foundation at the toe of the wall system. This becomes particularly limiting in tall vertical walls with sloping backfill. Conventionally designed cantilevered walls reduce the toe pressure by providing an arm perpendicular to and behind the wall face upon which the vertical load of the backfill acts, creating a moment opposite in direction to the moment due to the horizontal force of the backfill material on the wall face. This "moment" is increased for design purposes by increasing the area of the cantilever arm subject to the vertical loads by increasing the size or length of the moment arm until a suitable toe pressure is reached and a suitable factor of safety against overturning is reached, e.g., a factor of safety greater than 1.5. In other words, the resultant vertical force on the tieback lever arm which extends into the soil and the moment arm of this resultant vertical force about the toe of the wall acts is increased by increasing the length and horizontal surface area of the cantilevered arm until it is equal to 15 times the moment produced by the horizontal resultant force produced by the backfill on the inside wall face of the retaining wall. By reducing this "overturning moment," bearing pressures on the toe of the retaining wall system are decreased.
Many different schemes for increasing the opposing moment force, i.e., the vertical force on the lever arm, have been employed and are well known in the art. For example, British Pat. No. 1402 issued in 1908 to Walter E. Adams discloses a retaining wall structure having frames A which support wall panels B. The Adams device resists overturning by leverage due to the vertical resolved weight of the frame A. Adams discloses on page 1, line 20-25, that the greater the vertical force, the longer the leverage and the greater the resistance of the wall to the overturning moment.
U.S. Pat. No. 4,050,254 issued Sept. 27, 1977 to Meheen et al. discloses a similar system which achieves a safety factor for overturning by extending the lever arm into the soil backfill. This transmits the horizontal pressure on the retaining wall back into the overburden. The reinforcing web of the Meheen et al. patent forms a part of the unitary structure of the tieback element.
The disadvantages and limitations of static leverage walls such as disclosed in Adams and Meheen is that the base portions of the tieback elements must be considerably longer than the column portions which engage the wall panels in order to produce a factor of safety which is sufficient to overcome the overturning moment, i.e., the resultant horizontal force on the panels which is resolved into the column portion (vertical portion) of the tieback element. For example, Meheen et al. teaches the use of column beams 10 feet high and leg beams 28 feet long. Consequently a considerable cut must be made into the soil behind the retaining wall in comparison to the height of the soil retained for conventional static leverage retaining wall systems in order to meet suitable factors of safety. This design constraint effectively limits the height of a wall to single tiers 10 to 12 feet high. Higher walls can only be created by setting back subsequent tiers, as illustrated in the Meheen et al. patent.